This invention relates to the manufacture of expanded sheet stock and, more particularly, to an improved machine for cutting openings, such as diamond-shaped openings, in sheet stock by shearing portions of said stock in a staggered sequential manner.
Typically, expanded metal sheet or foil is produced by the use of a cutter blade having a serrated or tooth-like cutting edge. The blade is rectilinearly reciprocated toward a metal sheet or strip while the strip is clamped against a stationary, straight-anvil type blade. The reciprocation toward the strip is limited so that only a portion of the total depth of the teeth of the reciprocating blade passes through the metal. Following each reciprocation toward and away from the strip, the strip is indexed forward and the blade is rectilinearly reciprocated along its length 1/2 of a tooth spacing. Thus, the cut produced by the next reciprocation toward the strip is offset from the previous cut. In this manner, the double reciprocatory movement of the blade produces the well-known, standard expanded metal cut.
To increase the rate of expanded metal production, it has been the practice to increase the length of the blade and/or increase the rate of blade reciprocation. These approaches have been particularly satisfactory. First, with increases in blade length, it becomes somewhat difficult to assure sufficient blade rigidity to maintain the required close spacing and tolerances (less than 0.10 of the thickness of the metal being cut) between the reciprocating blade and the stationary blade, and to maintain parallelism between the blades. Moreover, increases in blade length ordinarily mean increases in the mass of the blade. This increases the problems involved in dynamically balancing the apparatus. As is conventional with reciprocating-type machines, the higher the speed at which they are operated, the more critical become balancing problems. Furthermore, it is difficult to maintain tolerances in the machine because of the high inertia generated by the blade at the limits of its stroke.
The foregoing problems complicate the maintenance of an extremely precise spacing and a relative relationship between the stationary and reciprocating blades. The criticality of the relative spacing becomes greater as the material thickness decreases and the blade speed increases, since it is necessary to space the blades within one-tenth of the thickness of the material being expanded. To meet this requirement, the prior art has used complicated blade mounting mechansims which were difficult to adjust and often did not stay in adjustment. These problems of blade adjustment had the further disadvantage of greatly increasing the time required to change blades. Also, the blade assemblies were often difficult to sharpen and total readjustment of the apparatus was required following each sharpening.
The prior art machines also employ a mechanically operated clamp to hold the strip during each cut. This clamp is released after each cut and the strip is advanced so that the next cut may be performed. As can be appreciated, the means used to clamp the strip during each cut must be exactly synchronized with blade movement so that the movable blade does not perform sequential cuts in a single plane, which would thereby sever the strip. The problems of synchronization, again, increase substantially with increases in blade speed because of the short time available for actuation and release of the clamping means.
The prior art machines mechanically index the sheet following each cutting stroke. Increases in the rate of blade reciprocation, therefore, increase the indexing rate of the feed mechanism. The prior art machines, therefore, index forward a slight amount following each reciprocation and then bring the strip to a complete stop during the time the actual cutting operation takes place. Therefore, as the blade reciprocates away from the strip, the strip must be accelerated and advanced the required distance, after which it must again be brought to a precise stop.
In the past, various types of roll feeding means having mechanical indexing arrangements interrelated with the blade drive have been used. Generally, with increased rate of speed, the problems of rapidly indexing the strip forward become almost insurmountable because of the inertia and lost motion inherent in mechanical indexing devices. As a result, it is not possible to stop and start feed rolls with the speed and accuracy required for producing a satisfactory product at the production rate desired. The problem is especially troublesome when it is desired to produce a fine mesh, expanded metal foil.